1. Field of the Invention
This invention relates to a method for monitoring the products of combustion, particularly in the area of hazardous waste combustion, using optical methods, namely infrared adsorption using tunable diode lasers. It also relates to the field of feedback process control.
2. Prior Art
Hazardous substances are materials which may pose a danger to living organisms, materials, structures, or the environment, by explosion or fire hazards, corrosion, toxicity to organisms, or other detrimental effects. Hazardous waste is a hazardous substance which has been discarded, abandoned, neglected, released, designated as a waste material, or one which may interact with other substances to be hazardous. Some examples of hazardous substances or hazardous waste include, aliphatic and aromatic solvents, chlorinated aliphatics, chlorinated aromatics, nitro-aliphatics, nitro-aromatics, materials containing heavy metals, aqueous-phase materials which contain traces of hazardous components, mixed hazardous wastes (organic solvents with radionuclides), mixtures of materials, and the like. Destruction of hazardous materials has become a major issue in recent years as conventional disposal methods such as land filling and deep well injection become less acceptable. The Resource Conservation and Recovery Act (RCRA) of 1976 makes technologies which mineralize hazardous waste (i.e. convert them to CO.sub.2, H.sub.2 O, HCl, and the like) more attractive. The RCRA statutes require that hazardous wastes in the category of principle organic hazardous components (POHCs) have destruction and removal efficiencies (DREs) of 99.99% or better. If the wastes contain chlorinated dibenzo-p-dioxins, chlorinated dibenzofurans, or chlorinated phenols, the EPA "dioxin rule" requires a DRE of 99.9999%.
There are several available technologies for hazardous waste destruction such as bio-degradation and plasma arc processing, but the most developed destruction method is incineration. A perfectly controlled incinerator can easily meet the mandated DREs because of both the thermodynamic driving force to convert the waste to benign products and the rapid chemical kinetics of oxidation. Real incinerators, however, cannot always be perfectly controlled and a current deficiency in incinerator technology is the lack of a rapid process monitor to detect periods in which combustion is less than ideal. If such a process monitor existed, it could be coupled into a feedback control system which would modify combustion conditions (e.g., decrease fuel flow rate) to bring the emissions back into design limits.
There are several possible causes for cases in which the target DREs are not met. First, the primary combustion zones in an incinerator are highly turbulent. Thus, there exists a non-zero statistical chance that a packet of fuel can pass through this zone without being well mixed with oxygen and oxidizing free radicals. Second, the colder walls of the incinerator can create a quench zone in which material is not fully oxidized. Lastly, there could be equipment failure or operator error which causes large quantities of hazardous waste to pass through the incinerator untreated or made more hazardous.
It is this last condition for which very rapid detection of the upset condition is required. For example, if the incinerator is treating waste covered by the "dioxin rule", then the entire annual "budget" for emissions is used up in 30 seconds. An on-line diagnostic method would also help convince the public that a waste incineration process is operating as designed. The present invention relates to a sensitive, diagnostic method for rapid, on-line monitoring of combustion processes in general and hazardous waste incineration in particular.
The present invention is believed to be an improvement over the present methods of monitoring combustion emissions. Present monitoring methods are limited to monitoring the flue or stack gases of combustion plants. This allows for less control over the combustion process and for a greater chance that hazardous materials will escape before the fault can be corrected. As discussed below, the method of the present invention is designed to monitor combustion emissions in the combustion chamber itself. Additionally, the method of the present invention preferably monitors compounds which are indicators of poor combustion efficiency, another improvement over the present monitoring methods.